For most of us, as we age, our brains shrink steadily, and if we live long enough, they end up resembling shrivelled walnuts, floating in a sea of cerebro-spinal fluid, confined within our skull. And yet we usually still feel that we are our true selves, albeit diminished, slow and forgetful. The problem is that our true self, our brain, has changed, and as we have changed with our brains, we have no way of knowing that we have changed.
